The present invention relates to steam turbine control, particularly for operation at low load levels.
During the operation of steam turbines, for example in boiler fired power plants, there is frequently a need to run the turbines at very low load levels, often as low as 5 to 10 percent of rated load, for extended periods of time. Since, however, such installations are designed to operate at rated load, operation at very low load levels entails a decrease in energy utilization efficiency. In addition, known procedures for operating at low load levels give rise to temperature transients which have an adverse effect on plant service life.
Steam turbines employed in power plants include a section containing a first stage to which steam is supplied via a plurality of inlet nozzles distributed around the entire, or a selected portion of the, periphery of the first stage housing. The supply of steam to the first stage can be effected in either one of two ways: full arc admission in which steam is supplied uniformly via all inlet nozzles; and partial arc admission in which the turbine control valves supplying selected inlet nozzles are closed in sequence to effect a progressive reduction in turbine output.
A turbine operating with full arc admission can be run at reduced load by a procedure known as the sliding pressure method in which the speed of the feed pump supplying water to the boiler is reduced. This reduces the pressure throughout the system starting at the pump outlet, through the boiler, the superheaters and, finally, the turbine stages. Reduction in pressure results in a corresponding reduction in the saturation temperature of the steam flowing through these components, so that a temperature transient is experienced by the drum and water walls thereof during each load reduction cycle.
In the case of partial arc turbines, turbine output is reduced first by sequentially closing selected turbine control valves. During this phase, normal pressure and temperature conditions are maintained throughout the system, although the quantity of steam being delivered to the turbine first stage is reduced.
This procedure is most efficient until a point is reached at which the quantity of steam being supplied to the first stage decreases to a certain proportion, in many systems when half of the control valves are fully open and half of the control valves are fully closed. It is more efficient to respond to further load reductions according to the sliding pressure method, as described above.
In the case of each of the procedures described above, reducing the boiler pressure imposes certain temperature stresses on the boiler, particularly since the steam saturation temperature decreases with pressure. Therefore, there has developed the practice of setting a lower limit value on boiler pressure. Once the lower limit value has been reached, further load reductions are handled by throttling, i.e., reducing the flow through, the open turbine control valves. Throttling creates certain disadvantages because it results in cooling of the steam due to the Joule-Thompson effect. This causes temperature transients to occur at the valve bodies and in the turbine. In addition, the steam experiences a loss of available energy which reduces the overall efficiency of the system.